As a fluid machine constituting a refrigeration cycle apparatus, a fluid machine 419, as shown in FIG. 7, in which a compression mechanism 402 for compressing a refrigerant and an expansion mechanism 404 for converting expansion energy of a refrigerant with its decompression and expansion into mechanical power are integrated as a single unit, has been known (JP 62(1987)-77562 A). By supplying the mechanical power obtained by the expansion mechanism 404 to the compression mechanism 402 by a shaft 405, the efficiency of the refrigeration cycle apparatus is enhanced.
Since the compression mechanism 402 compresses the refrigerant adiabatically, the temperature of the components of the compression mechanism 402 rises as the temperature of the refrigerant rises. On the other hand, since the expansion mechanism 404 draws the refrigerant which has been cooled by a radiator and expands the drawn refrigerant adiabatically, the temperature of the components of the expansion mechanism 404 falls as the temperature of the refrigerant falls. Accordingly, in the case where the compression mechanism 402 and the expansion mechanism 404 are integrated simply as shown in FIG. 7, the heat of the compression mechanism 402 transfers to the expansion mechanism 404, and thereby the expansion mechanism 404 is heated and the compression mechanism 402 is cooled. In this case, as shown by arrows in a Mollier diagram of FIG. 8A, the enthalpy of the refrigerant discharged from the compression mechanism 402 decreases and the heating capacity of the radiator is reduced in a real cycle, compared with that in a theoretical cycle. Furthermore, the enthalpy of the refrigerant discharged from the expansion mechanism 404 increases and the refrigerating capacity of the evaporator is reduced.
Especially in the case of a water heater, water needs to be heated by the radiator to a preset temperature of stored hot water. Therefore, it must be ensured that the temperature of the refrigerant discharged from the compression mechanism is higher than the preset temperature of the stored hot water. However, when a thermal short-circuit occurs between the compression mechanism and the expansion mechanism, the temperature of the refrigerant discharged from the compression mechanism drops, which causes insufficient heating of water and thus reduces the temperature of the stored hot water to a temperature lower than the preset one. One of the methods for compensating the drop in the temperature of the refrigerant discharged from the compression mechanism caused by this thermal short-circuit is a method for raising the pressure of the refrigerant discharged from the compression mechanism, as in the theoretical cycle of discharge temperature control shown in FIG. 8B. Specifically, the temperature of the discharged refrigerant is raised by compressing the refrigerant somewhat excessively. By doing so, the reduction in the heating capacity caused by the thermal short-circuit can be compensated, as in the real cycle of discharge temperature control shown in FIG. 8B. This method, however, imposes an excessive workload on the compression mechanism. Therefore, the power consumption of the motor increases, which reduces the effect of recovering mechanical power in the expansion mechanism.
One of the means for solving this problem is a configuration, as shown in FIG. 9, in which the internal space of a closed casing 501 is filled with a low-pressure refrigerant guided from an evaporator into a compression mechanism 502, and the compression mechanism 502 and an expansion mechanism 504 are spaced apart from each other (JP 2005-264829 A).
Another means for solving this problem is a configuration, as shown in FIG. 10, in which the internal space of a closed casing 601 is partitioned into a low-pressure space 652 and a high-pressure space 651, an expansion mechanism 602 is disposed in the low-pressure space 652 and a compression mechanism 604 is disposed in the high-pressure space 651, and a refrigerant to be drawn into the compression mechanism 604 is guided into the low-pressure space 652 and a refrigerant discharged from the compression mechanism 604 is guided into the high-pressure space 651 (JP 2006-105564 A).
With the configuration shown in FIG. 9, the surrounding space of the expansion mechanism 504 is filled with the refrigerant to be drawn into the compression mechanism 502. Therefore, heat transfer from the refrigerant in the closed casing 501 to the expansion mechanism 504 can be suppressed. Heat transfer also occurs between the compression mechanism 502 and the refrigerant to be drawn thereinto. However, the refrigerant that has received heat from the compression mechanism 502 is compressed by the compression mechanism 502 and thereby heats the compression mechanism 502 itself. Thus, the temperature of the refrigerant discharged from the compression mechanism 502 does not drop.
However, in the configuration in which the internal space of the closed casing 501 is filled with the low-pressure refrigerant, the refrigerant discharged from the compression mechanism 502 is discharged directly to the refrigeration cycle (refrigerant circuit) through a discharge pipe 509. Therefore, the amount of oil discharged to the refrigeration cycle increases, compared to the configuration in which the internal space of the closed casing 501 is filled with the refrigerant discharged from the compression mechanism 502. The discharged oil adheres to a refrigerant pipe and increases pressure loss, or degrades the performance of the radiator and the evaporator.
On the other hand, with the configuration shown in FIG. 10, the refrigerant discharged from the compression mechanism 604 is released once into the high-pressure space 651 of the closed casing 601 and thereafter discharged to the radiator through a discharge pipe 609 of the high-pressure space 651. Since the oil is separated from the refrigerant discharged from the compression mechanism 604 in the internal space of the closed casing 601, the refrigerant discharged from the compression mechanism 604 can be prevented from circulating through the refrigeration cycle, together with a large amount of oil.
The fluid machine shown in FIG. 10, however, has a configuration in which the internal space of the closed casing 601 is partitioned into the low-pressure space 652 and the high-pressure space 651, and therefore the shaft 605 for coupling the expansion mechanism 602 and the compression mechanism 604 needs to penetrate a partitioning member 650. In this case, a mechanical seal must be used to prevent the leakage of the refrigerant from a clearance between the shaft 605 and the partitioning member 650, which causes a concern about an increase in friction loss.